NASA's Chandra X-ray Observatory has taken deep exposures of two nearby energetic pulsars flying through the Milky Way galaxy. The shape of their X-ray emission suggests there is a geometrical explanation for puzzling differences in behavior shown by some pulsars.

Pulsars - rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars
were discovered 50 years ago via their pulsed, highly regular, radio
emission. Pulsars produce a lighthouse-like beam of radiation that
astronomers detect as pulses as the pulsar's rotation sweeps the beam
across the sky.

Since their discovery, thousands of pulsars have been discovered,
many of which produce beams of radio waves and gamma rays. Some pulsars
show only radio pulses and others show only gamma-ray pulses. Chandra
observations have revealed steady X-ray emission from extensive clouds
of high-energy particles, called pulsar wind nebulas, associated with
both types of pulsars. New Chandra data on pulsar wind nebulas may
explain the presence or absence of radio and gamma-ray pulses.

This four-panel graphic
shows the two pulsars observed by Chandra. Geminga is in the upper left
and B0355+54 is in the upper right. In both of these images, Chandra's
X-rays, colored blue and purple,
are combined with infrared data from NASA's Spitzer Space Telescope
that shows stars in the field of view. Below each data image, an
artist's illustration depicts more details of what astronomers think the
structure of each pulsar wind nebula looks like.

For Geminga, a deep Chandra observation totaling nearly eight days
over several years was analyzed to show sweeping, arced trails spanning
half a light year
and a narrow structure directly behind the pulsar. A five-day Chandra
observation of the second pulsar, B0355+54, showed a cap of emission
followed by a narrow double trail extending almost five light years.

The underlying pulsars are quite similar, both rotating about five
times per second and both aged about half a million years. However,
Geminga shows gamma-ray pulses with no bright radio emission, while
B0355+54 is one of the brightest radio pulsars known yet not seen in
gamma rays.

A likely interpretation of the Chandra images is that the long narrow
trails to the side of Geminga and the double tail of B0355+54 represent
narrow jets emanating from the pulsar's spin poles. Both pulsars also
contain a torus, a disk-shaped region of emission spreading from the
pulsar's spin equator. These donut-shaped structures and jets are
crushed and swept back as the pulsars fly through the Galaxy at
supersonic speeds.

In the case of Geminga, the view of the torus is close to edge-on,
while the jets point out to the sides. B0355+54 has a similar
structure, but with the torus viewed nearly face-on and the jets
pointing nearly directly towards and away from Earth. In B0355+54, the
swept-back jets appear to lie almost on top of each other, giving a
doubled tail.

Both pulsars have magnetic poles quite close to their spin poles, as
is the case for the Earth's magnetic field. These magnetic poles are the
site of pulsar radio emission so astronomers expect the radio beams to
point in a similar direction as the jets. By contrast the gamma-ray
emission is mainly produced along the spin equator and so aligns with
the torus.

For Geminga, astronomers view the bright gamma-ray pulses along the
edge of the torus, but the radio beams near the jets point off to the
sides and remain unseen. For B0355+54, a jet points almost along our
line of sight towards the pulsar. This means astronomers see the bright
radio pulses, while the torus and its associated gamma-ray emission are
directed in a perpendicular direction to our line of sight, missing the
Earth.

These two deep Chandra images have, therefore, exposed the spin
orientation of these pulsars, helping to explain the presence, and
absence, of the radio and gamma-ray pulses.

The Chandra observations of Geminga and B0355+54 are part of a large
campaign, led by Roger Romani of Stanford University, to study six
pulsars that have been seen to emit gamma-rays. The survey sample covers
a range of ages, spin-down properties and expected inclinations, making
it a powerful test of pulsar emission models.

A paper on Geminga led by Bettina Posselt of Penn State University
was accepted for publication in The Astrophysical Journal and is available online.
A paper on B0355+54 led by Noel Klingler of the George Washington
University was published in the December 20th, 2016 issue of The
Astrophysical Journal and is available online.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts,
controls Chandra's science and flight operations.